This invention relates to an internal combustion engine of the stratified-charge type to reduce harmful substances present in exhaust emissions.
Various methods have been proposed to reduce unburned harmful substances such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) that are present in emissions of internal combustion engines. Among these engines is the stratified-charge engine that burns lean air-fuel mixtures. This is one of the most effective engines taking advantage of the fact that a mixture of a high air-fuel ratio, when combusted, leaves less harmful substances, particularly NOx, in exhaust emissions.
Due to its poor ignitability and low flame-propagation rate, a lean air-fuel mixture usually lowers thermal efficiency of the cycle and fails to produce large torque. Therefore, it was proposed to provide an engine having an auxiliary combustion chamber in addition to a main combustion chamber, so that the lean air-fuel mixture in the main combustion chamber can be ignited by the flame developed in the auxiliary combustion chamber. In one type of such engine, a richer mixture is fed through an auxiliary intake valve into the auxiliary combustion chamber where it is ignited by a spark plug; then a lean mixture in the main combustion chamber is burned by the flame propagating from the auxiliary combustion chamber. In another type, the auxiliary combustion chamber is disposed in the vicinity of the air-intake port, and a rich air-fuel mixture supply device, without an auxiliary intake valve, is provided to feed a rich mixture into the auxiliary combustion chamber during the intake stroke.
These conventional engines have the disadvantage of poor ignitability since the electric arc produced by the spark plug is blown out by the stream of air-fuel mixture forced from the auxiliary combustion chamber into the main combustion chamber during the combustion stroke. Also, such engines have been costly because of the need to provide an auxiliary fuel supply device, such as carburetor, to supply a rich mixture into the auxiliary combustion chamber.